The American lobster Homarus americanus supports a valuable commercial fishery in the Northeastern USA and Maritime Canada; however, stocks in the southern portion of the lobster's range have shown declines, in part due to the emergence of shell disease. Epizootic shell disease is a bacterially induced cuticular erosion that renders even mildly affected lobsters unmarketable because of their appearance, and in more severe cases can cause mortality. Despite the importance of this disease, the associated bacterial communities have not yet been fully characterized. We sampled 2 yr old, laboratory-reared lobsters that displayed signs of shell disease at the site of disease as well as at 0.5, 1, and 1.5 cm away from the site of disease to determine how the bacterial community changed over this fine spatial scale. Illumina sequencing of the 16S rRNA gene revealed a distinct bacterial community at the site of disease, with significant reductions in bacterial diversity and richness compared to more distant sampling locations. The bacterial community composition 0.5 cm from the site of disease was also altered, and there was an observable decrease in bacterial diversity and richness, even though there were no signs of disease at that location. Given the distinctiveness of the bacterial community at the site of disease and 0.5 cm from the site of disease, we refer to these communities as affected and transitionary, and suggest that these bacteria, including the previously proposed causative agent, Aquimarina 'homaria', are important for the initiation and progression of this laboratory model of shell disease.KEY WORDS: American lobster · Shell disease · Bacterial communities · Microbiome · High throughput sequencing · 16S rRNA gene Resale or republication not permitted without written consent of the publisherDis Aquat Org 124: [41][42][43][44][45][46][47][48][49][50][51][52][53][54] 2017 2005). Due to the physical severity of epizootic shell disease, even mildly affected lobsters are generally considered unmarketable due to the unappealing appearance of their shells, thus affecting the value of this important commercial fishery .In addition to aesthetic changes associated with epizootic shell disease, animals can also suffer from decreased health ) and increased mortality . Lobsters affected by shell disease are energetically compromised, with disruptions to their metabolism and hormone signaling (Tarrant et al. 2012). They also tend to have high levels of the molting hormone ecdysone, which can disrupt their reproductive cycle (Laufer et al. 2005). These physiological changes may in turn influence population dynamics, particularly since disease prevalence among egg-bearing females can be as high as 70% (Castro & Somers 2012). The recent decline in the southern New England American lobster population has been positively associated with an in crease in epizootic shell disease , Howell 2012, highlighting the importance of understanding the ecological dynamics of this disease.Despite the importance of epizootic ...

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